The present invention relates to a crosspoint switch, and in particular to an apparatus and a method suitable for providing alterative pathways through the switch in the event of a failure of a switching element. These alternative pathways have the potential to be engaged automatically and/or remotely.
Communications networks are moving towards becoming all optical (photonic) networks, incorporating photonic (optical) switching in which optical signals are switched directly rather than converted to electrical signals, switched electrically, then converted back to optical signals for re-transmission. Photonic switches may be used to switch wavelength division multiplexed (WDM) signals as a group, or the WDM signals may be demultiplexed outside the switch and switched individually as channels, or as groups of channels as desired. Photonic switches are fabricated using a range of technologies and frequently employ a crosspoint (crossbar) architecture. In such architectures light from an input port may traverse a number of switching elements. At each switching element the light may be switched and directed towards an associated output port or alternatively pass though to the next switching element. Once the light has been directed towards an output port it may traverse more switching elements which, in most implementations, must remain inactive so as not to block or disrupt the light path before it reaches its output port.
For example, a recently developed photonic switch using Micro Electro-Mechanical systems (MEMS) technology is described in xe2x80x9cFree-Space Micro Machined Optical Switches for Optical Networkingxe2x80x9d by L Y Lin et al, IEEE Journal of Selected Topics In Quantum Electronics, Vol. 5 No.1, January/February 1999; which is incorporated herein by reference. Such MEMS switches typically use moveable mirrors to redirect optical paths within the switch in order to complete an optical signal or channel connection across the switch.
FIG. 1 shows a schematic diagram of a typical MEMS photonic switch 100. The switch 100 is bi-directional, but for simplicity is assumed to comprise 4 inputs in the form of optical fibres 112, 114, 116 and 118, and 4 outputs which are also optical fibres 122, 124, 126 and 128 Each input and output has an associated lens 104 which collimates the beams from the inputs and focuses the beam at the outputs. Such a switch is generically referred to as a 4xc3x974 switch (number of inputsxc3x97number of outputs).
The switch 100 is a crosspoint (cross bar) switch, having a switching element (here, a mirror, 106) located at each of the points at which optical signals emitted from the input fibres would cross with optical signals emitted from the output fibres. The switch 100 thus has a four by four array of mirrors 106 mounted on a surface 102.
In this particular switch, each mirror may be moved between two stable positions. FIGS. 2a and 2b illustrate these positions. FIG. 2a shows the mirror in the inactivated position 106a, where the mirror is flat i.e. substantially parallel to the surface 102. FIG. 2b shows the mirror having been raised to the activated or upright position 106b, substantially perpendicular to the surface 102. This activation may be performed by a variety of means e.g. by micro actuators causing the mirror to be rotated about the hinges 108. The mirrors are typically formed of materials such as polysilicon, the reflectivity of which is increased by providing a reflective coating 107 such as gold. In the inactivated state, it is typical for the relatively non-reflective surface 109 of the mirror to lie adjacent to the surface 102, so that the reflective coating 107 does not contact the surface 102.
FIG. 1 shows a typical operation of the switch 100. By raising the appropriate mirrors, an optical signal from each of the inputs 112, 114, 116 and 118 is directed to a respective output 128, 126, 122 and 124. For instance, an optical signal originating from input fibre 112 is formed into a collimated beam 132 by lens 104. The beam 132 then reflects off the front reflective surface 107 of a raised mirror 106b into a further lens 104 which focuses the beam 132 into the output fibre 128. it will be appreciated that by appropriate control of the array of mirrors 106, any one of the signals originating from the inputs 112, 114, 116 and 118 can be switched into any one of the outputs, 122, 124,126 and 128.
In any system switching information, it is desirable to provide alternative pathways for the information in the event that the original pathway xe2x80x9cfailsxe2x80x9d and is unable to transmit the signals as desired. Such alternative pathway provision is commonly referred to as xe2x80x9cprotectionxe2x80x9d when these pathways may be engaged remotely and/or automatically.
it will be appreciated that a failure in any of the internal switching elements (mirrors 106) would impair the functionality of the switch. For instance, any of the mirrors could be jammed in either the raised 106b or flat 106a position, and this would prevent a connection between the input and output corresponding to that mirror. In addition a mirror which is jammed in the raised position has the potential to prevent a connection between the associated input and another output and between another input and the associated output. This is because the raised mirror may act as a block to such light paths.
The present invention aims to address such problems.
In a first aspect, the present Invention provides a crosspoint switch comprising N primary inputs, M primary outputs and an array of (N+X)xc3x97(M+X) switching elements, where M, N and X are all positive Integers, the additional switching elements in said array being arranged to provide alternative connectivity between said inputs and outputs.
A typical crosspoint switch having N inputs and M outputs will have an array of Nxc3x97M switching elements. By providing the additional switching elements in the array, it becomes possible to provide alternative connectivity between the inputs and outputs to compensate for any failures in the part of the array normally utilised for switching.
Preferably, said switch is a photonic switch. Photonic switches can have switching elements such as reflective surfaces (mirrors), refractive media, or interferometers.
Preferably, said additional switching elements comprise at least one column at an outermost edge of the array, and at least one row at an outermost edge of the array.
Preferably, the additional switching element located at the intersection of each of said row and said column is located in a fixed position so as to redirect incident signals in a predetermined manner. Such a switching element can act to redirect an incident signal from said row along said column, or from said column along said row.
Alternatively the switch can further comprise X additional inputs and X additional outputs, each of said additional outputs being transmissively connected to a respective additional input.
Preferably, said switch is a photonic switch, and said additional outputs are connected to said additional inputs by an optical fibre or other form of optical wave-guide.
Optionally, at least one of said additional outputs is coupled to a tap for the monitoring of signals passing through said output.
Preferably, N=M.
Preferably, X=2. If X=2, or any even number, protection can be provided for one or more switching elements that fail in the active position and act to block signals.
Preferably, said array is substantially rectilinear.
In another aspect the present invention provides a node for a telecommunications network comprising a crosspoint switch comprising N Inputs, M outputs and an array of (N+X)xc3x97(M+X) switching elements, where M, N and X are all positive integers, the additional switching elements in said array being arranged to provide alternative connectivity between said inputs and outputs.
In another aspect the present invention provides a transmission system comprising a transmitter and a receiver, and a transmission line connecting the transmitter to the receiver, the system further comprising a crosspoint switch comprising N inputs, M outputs and an array of (N+X)xc3x97(M+X) switching elements, where M, N and X are all positive integers, the additional switching elements in said array being arranged to provide alternative connectivity between said inputs and outputs.
In a further aspect the present invention provides a method of operating a crosspoint switch comprising N inputs, M outputs and an array of (N+X)xc3x97(M+X) switching elements, where N, M and X are all positive integers, the method comprising detecting that switching element has ceased to function correctly; and
providing control signals to the switching elements for configuring the switching elements to provide the same connectivity as the incorrectly functioning switching element.
Preferably, said switch is a photonic switch arranged to switch optical signals, the method further including the step of providing control signals to the switching elements for configuring the switching elements so as to ensure that no optical signals are blocked by the incorrectly functioning switching element.
In another aspect, the present invention provides a computer program arranged to perform the method of a method of operating a crosspoint switch comprising N inputs, M outputs and an array of (N+X)xc3x97(M+X) switching elements, where N, M and X are all positive integers, the method comprising detecting that switching element has ceased to function correctly; and providing control signals to the switching elements for configuring the switching elements to provide the same connectivity as the incorrectly functioning switching element.
Preferably, said computer program is stored on a machine readable medium.